Abstract:

The dynamics of the East Madagascar Current (EMC)
system, and its influence
on the biological production associated to the
shelf, is studied by using cruise-data
collected onboard the Norwegian R/V. Dr. Fridtjof
Nansen between August and October,
2008. The South Equatorial Current (SEC) hits the
coast of Madagascar at 19°S, bifurcating into a
northwards and southwards directed branch of the
EMC.
The southwards directed branch behaves like a
strong (up to 150 cms−1) western
boundary current, with a volume transport of ~26
Sv at 25°S. The volume transport
was calculated using geostrophic current profiles
adjusted with ADCP measurements
at 176 meters depth. At the south-eastern corner
of the island, some of the EMC
continues south-westwards, and some is
retroflected north-eastwards. Cyclonic eddies
are expected to be generated inshore of the EMC,
while the anticyclonic eddies
seem to the shed off the retroflected part of the
current.
The currents of the northwards directed branch of
the EMC are weaker (up to
60 cms−1) just north of the bifurcation. The
volume transport increases between
13.5°S and 12°S. This could be explained by
another incoming branch of the SEC.
The north-westwards directed volume transport
north of the island (12°S) is ~62
Sv. The wind distribution seems to be the most
important factor explaining the
current system at the eastern coast.
Wind and current driven upwelling seems to be the
most important physical factor
stimulating the biological production of the
southern and south-eastern coasts. The
south-eastern corner of the island could be
considered as a biological hot-spot, where
a divergent current field contributes to the
upwelling. Discharge of nutrient rich
water from rivers seems to contribute to the
production at the north-eastern coast.
The width of the shelf has also turned out to be
an important factor.